1. Field of the Invention
The invention relates to a magnetic resonance (MR) imaging method for imaging a part of a human or animal body arranged in a steady magnetic field, which method involves the use of imaging pulse sequences for the measurement of the MR signals for reconstructing the MR images, approximation of the part of the body by an ellipsoid, imaging of an imaging plane of the part which contains a minor axis of the ellipsoid, which minor axis is derived from a major axis of the ellipsoid, and which method includes the following steps for estimating the major axis:
a) formation of a first MR image of a first imaging plane and a second MR image of a second imaging plane of the part of the body, the first and the second imaging plane being chosen so as to extend transversely to a common axis of the body, PA1 b) determination of a first reference from the first MR image and a second reference from the second MR image, and PA1 c) determination of a first estimate of the major axis of the ellipsoid from a connecting line through two points which correspond to the first reference and the second reference, respectively. PA1 formation of a third MR image of a third imaging plane and a fourth MR image of a fourth imaging plane of the part of the body, the third and the fourth imaging plane being chosen so as to extend transversely to the first estimate made for the major axis, and a step in which a second estimate is made for the major axis by repeating the steps b) and c) while replacing the first MR image by the third MR image and the second MR image by the fourth MR image. This step yields a second estimate of the major axis of the ellipsoid which is a more accurate approximation than the first estimate of the major axis, thus reducing the deviation between the imaging plane reproduced by the MR image and the imaging plane containing the actual position of the minor axis. PA1 determination of an orientation line through a first point which corresponds to the first reference, substantially perpendicularly to the imaging planes, and PA1 determination of the next estimate of the major axis from a combination of a predetermined ratio of the minor axis to the major axis of the ellipsoid and an angle between the previous estimate of the major axis and the orientation line. The use of anatomic properties of the heart, for example the approximation of the left ventricle of the heart by means of an ellipsoid of revolution having the predetermined geometric ratio of the minor axis and the major axis, offers a more accurate estimate of the major axis.
The invention also relates to an MR device for carrying out such a method. An imaging plane is to be understood to mean herein a slice in the body of which an MR image is made.
2. Description of Related Art
A method of this kind is known from U.S. Pat. No. 5,107,838. The known method is used to form MR images for functional analysis of a part of the body, for example the heart of a human or animal body. It is thus used, for example to form perfusion MR images of the heart or MR images of the coronary artery system around the heart. According to the known method, the ellipsoid is used as a model description of, for example the left ventricle of the heart. Furthermore, for the formation of perfusion MR images or MR images of a coronary artery of the heart it is important that an imaging plane of the heart is reproduced which contains the minor axis of the ellipsoid. Because in every body the heart may be oriented differently with respect to a common axis of the body, for example a head to feet axis, the major axis and the minor axis to the ellipsoid must be determined relative to an axis transversely of the head-feet axis so as to form the MR image of the imaging plane containing the minor axis of the ellipsoid. The known method utilizes the two MR images so as to determine an orientation of the major axis, for which an operator determines, by way of a visual estimate, the first reference from the first MR image and the second reference from the second MR image. For the first reference, for example, a point in the first MR image is selected which corresponds to a point at an edge of a valve of the heart. For the second reference, for example, a point in the second MR image is chosen which corresponds to a point near a tip of the left ventricle. The major axis is subsequently determined by the connecting line between said points. The minor axis is subsequently determined from the major axis by way of an empirically determined ratio of the major axis to the minor axis of a human heart. It is a drawback of the known method that the estimate of the major axis is inaccurate and yields an MR image of an imaging plane of the body which deviates from an imaging plane of the body which contains the actual minor axis.